Laser induced breakdown of a material causes chemical and physical changes, chemical and physical breakdown, disintegration, ablation, and vaporization. Lasers provide good control for procedures which require precision such as inscribing a micro pattern. Pulsed rather than continuous beams are more effective for many procedures, including medical procedures. A pulsed laser beam comprises bursts or pulses of light which are of very short duration, for example, on the order of 10 nanoseconds in duration or less. Typically, these pulses are separated by periods of quiescence. The peak power of each pulse is relatively high often on the order of gigawatts and capable of intensity on the order of 1013 w/cm2. Although the laser beam is focused onto an area having a selected diameter, the effect of the beam extends beyond the focused area or spot to adversely affect peripheral areas adjacent to the spot. Sometimes the peripheral area affected is several times greater than the spot itself. This presents a problem, particularly where tissue is affected in a medical procedure. In the field of laser machining, current lasers using nanosecond pulses cannot produce features with a high degree of precision and control, particularly when nonabsorptive wavelengths are used.
It is a general object to provide a method to localize laser induced breakdown. Another object is to provide a method to induce breakdown in a preselected pattern in a material or on a material.
U.S. Pat. No. 5,656,186 to Mourou et al. is directed to a method for laser-induced breakdown. The teaching of Mourou et al. requires that the laser beam be focused to a point at or beneath the surface from which material is to be removed. Applicants have discovered that it is undesirable to focus at or beneath the surface since this results in undesired damage beneath the surface, in particular to the substrate (or underlayer) on which the material that is to be removed is disposed. This is particularly a problem where the underlayer is very sensitive to the laser light and/or can be easily damaged by the laser light. Applicants have unexpectedly discovered that the light source should be focused above the surface to be removed toward this undesired change.